Alright! Imagine a magnetic field that’s moving across or perpendicular to the direction something else is traveling — that’s basically a transverse magnetic field.

More technically: A transverse magnetic (TM) field means the magnetic field is entirely perpendicular to the direction of wave propagation. For instance, the magnetic field in a waveguide or electromagnetic wave is entirely “sideways” and has no component in the direction the wave is traveling.

The term “transverse” simply indicates that it points in the opposite direction of the direction that something else—such as a wave or particle—is traveling.